The purpose of the proposed experiments is to provide a detailed analysis of the functions of different subpopulations of neurons in the visual cortex which use the inhibitory neurotransmitter gama- amino butyric acid (GABA). We have produced two monoclonal antibodies, VC1.1 and VC5.1 that identify unique cell surface molecules restricted to subsets of GABAergic neurons in the visual cortex. We now propose to use these and new antibodies to study the functions of subpopulations of GABAergic neurons and to investigate the role of the cell surface molecules themselves in the formation of identified cortical circuits. To achieve these goals, we shall use immunological methods in combination with biochemical, anatomical, immunohistochemical and functional approaches. We shall define some of the biochemical characteristics of the cell surface molecules recognized by VC1.1 and VC5.1 and purify these molecules for functional assays. We shall determined whether the VC1.1 immunoreactive/GABAergic neurons in the visual cortex have other biochemical or anatomical features which provide clues to their function in cortical circuits. We shall determine whether extrinsic factors regulate the levels of VC1.1 or VC5.1 immunoreactive molecules by studying their expression in visual cortex following subcortical lesions or in cortical development and test the role of these molecules on cell survival, process outgrowth and synaptogenesis in vitro. As a foundation for future physiological analyses of the contribution of GABAergic neurons to the response properties of cortical neurons, we propose to immuno-ablate the VC1.1 or VC5.1 /GABAergic subpopulation of cortical neurons at different times during development and in mature animals. In other experiments, we shall obtain purified populations of VC1.1. positive neurons using fluorescence activated cell sorting and use these as immunogens to produce additional antibodies against subsets of cortical interneurons. Ultimately, these studies will aid further molecular and cellular analyses of cortical microcircuitry and lead to a more complete understanding of how cortical functions are disrupted by disease or congenital birth defects.